1. Field of the Invention
The present invention relates to an arrangement of color filters for an image sensor in which pixels have the offset sampling structure.
2. Related Background Art
FIGS. 1, 2 and 3 show conventionally known examples of an arrangement of color filters for a color solid-state image sensing device. In FIG. 1, green light transmissive filters (hereinafter referred to as "Gr filters") are vertically arrayed in the stripe form. Red light transmissive filters (hereinafter referred to as "Rd filters") and blue light transmissive filters (hereinafter referred to as "Bl filters") are arrayed two rows apart vertically and one column apart horizontally between the Gr filters.
In FIG. 2, magenta light transmissive filters (hereinafter referred to as "Mg filters"), green light transmissive filters, cyan light transmissive filters (hereinafter referred to as "Cy filters"), and yellow light transmissive filters (hereinafter referred to as "Ye filters") are arranged in the sequence as shown in units of eight color filters consisted of horizontal two pixels and vertical four pixels.
FIG. 3 is concerned with a solid-state image sensor having the offset sampling structure disclosed in Japanese Patent Application No. 1-24433, for example. In this offset sampling structure, Rd, Gr and Bl filters are arranged in units of three color filters consisted of horizontal three pixels and vertical one pixel with an offset amount of 1.5 pixels in the horizontal direction between two rows.
FIGS. 4, 5 and 6 are characteristic diagrams of the first quadrant as obtained by expressing color light carriers in the color filter arrangements of FIGS. 1, 2 and 3 using the two-dimensional frequency plane (f.sub.H, f.sub.V), respectively. Assuming that the horizontal pixel pitch is P.sub.H and the vertical pixel pitch is P.sub.V, each diagram represents an area of 0.ltoreq.f.sub.H .ltoreq.1/P.sub.H and 0.ltoreq.f.sub.V .ltoreq.1/2P.sub.V. In any diagram, the arrow indicates a carrier of each color, the arrow length indicates a magnitude of the carrier, and the arrow direction indicates a phase relationship.
In FIG. 4, color light carriers occur at (1/2P.sub.H, 0), (1/P.sub.H, 0), (0, 1/4P.sub.V), (1/2P.sub.H, 1/4P.sub.V) and (1/P.sub.H, 1/4P.sub.V) other than (0, 0). Among then, (0, 0) and (1/P.sub.H, 0) represent the carriers which occur for achromatic light and cause turn-back distortions. The remaining carriers are perfectly canceled out and disappeared for achromatic light, but not disappeared for chromatic light and cause turn-back distortions.
Likewise, in FIG. 5, color light carriers occur at (1/2P.sub.H, 0), (1/P.sub.H, 0), (1/2P.sub.H, 1/4P.sub.V), (0, 1/2P.sub.V), (1/2P.sub.H, 1/2P.sub.V) and (1/P.sub.H, 1/2P.sub.V) other than (0, 0). Among then, (0, 0) and (1/P.sub.H, 0) represent the carriers which occur for achromatic light and others represent the carriers which are disappeared for chromatic light.
In FIG. 6, assuming that the horizontal offset amount of the solid-state image sensor is P.sub.H /2, color light carriers occur at (2/3P.sub.H, 0), (1/3P.sub.H, 1/2P.sub.V) and (1/P.sub.H, 1/2P.sub.V) Among them, (0, 0) and (1/P.sub.H, 1/2P.sub.V) represent the carriers which occur for achromatic light and others represent the carriers which are disappeared for chromatic light.
It is generally known that the above offset sampling structure has a feature as follows. With the rectangular sampling structure as shown in FIGS. 1 and 2, since the carrier occurs for achromatic light at the frequency corresponding to the horizontal position of (1/P.sub.H, 0), f.sub.H =1/2P.sub.H becomes the Nyquist's frequency and the frequency component thereabove cannot be obtained. Thus, the horizontal resolution obtainable with this structure is maximally f.sub.H =1/2P.sub.H. On the other hand, with the offset sampling structure as shown in FIG. 3, the carrier does not occur for achromatic light at the frequency corresponding to the horizontal position of (1/P.sub.H, 0) and f.sub.H =1/P.sub.H can become the Nyquist's frequency. Therefore, although the color filter arrangement of FIG. 3 has the same sampling pitch as the rectangular sampling structure of FIGS. 1 and 2, it can realize the horizontal resolution twice the above cases, i.e., f.sub.H =1/P.sub.H.
However, because objects to be usually photographed are not always achromatic but colored in general cases, the color light carriers at all the positions shown in FIGS. 4, 5 and 6 generate turn-back distortions, so that some scenes may be awkward or hard to see. This necessitates use of an optical low-pass filter or the like to cut the detrimental color light carriers, which leads to a reduction in the resolution.
In the color solid-state image sensing device with the offset sampling structure shown in FIGS. 3 and 6, for example, the color light carrier occurs at the position of (2/3P.sub.H, 0) and, therefore, an optical low-pass filter capable of cutting off the frequency component above f.sub.H =2/3P.sub.H in the horizontal direction is required. This means that while the horizontal resolution up to f.sub.H =1/P.sub.H could be intrinsically obtained for achromatic light, the practically obtainable horizontal resolution is only 2/3of that, i.e., f.sub.H =2/3P.sub.H.